The invention relates to a blown film extrusion plant comprising a blow head and an air cooling ring and to a method for operating a blown film extrusion plant.
In blown film extrusion, the thermoplastic melt provided by the extruder is shaped in the blow head into a hose. Following exit, the shaped hose is blown up with air from the inside. At the upper end of the film bubble, the inflated film hose is continuously laid flat by suitable guide mechanisms and drawn off by a roll pair.
The molten film hose, from its exit from the nozzle, is intensively cooled with air. This is done with the aid of an air cooling ring, which blows cooling air radially onto the hose from the outside. For the stabilization of the film hose in the turbulent air current, this is led, higher up, through a sizing cage. In blown film plants, an additional internal air cooling with air exchange has nowadays become established. Under the influence of the cooling, the passage of the film from the molten into the solid state is realized, which is achieved at the frost line.
As a result of blow air being blown into the interior of the hose, the molten film hose is widened to the desired diameter. Inside the film hose, a constant slight overpressure prevails. The quotient of the end diameter of the hose and the exit diameter of the blow head is here referred to as the blow-up ratio. At the same time, for the transverse stretching, the hose is longitudinally stretched at the upper end to the desired film thickness by the draw-off rolls, at an increased speed relative to the melt exit speed. The quotient of draw-off speed and exit speed here denotes the draw-off ratio. As a result of the described and concurrent transverse and longitudinal stretching of the film hose, a biaxial elongation of the plastic melt thus takes place following departure from the blow head.
The two variables blow-up and draw-off ratio substantially characterize the influence of the process upon the mechanical film properties of the produced blown film. Apart from the molecular properties, which determine the mechanical properties of a blown film and which are different for each individual polymer, it is namely, in particular, the extent of the stretching and thus the degree of orientation which are responsible for the mechanical properties of a blown film. With increasing orientation, that means with increasing biaxial elongation in the hose-forming zone, the mechanical film properties, such as, for example, tensile strength, can in this case be enhanced.
The generation of the molecular orientation by the biaxial stretching in the hose-forming zone is not, however, irrevocable. Owing to the very strong mobility of the macromolecules which exists at the prevailing temperatures in the hose-forming zone, a part of the orientation is re-formed, already during the stretching, by the orientation relaxation. The orientation relaxation has the effect that the achievable orientation state is not only dependent on the degree of elongation, but is dismantled all the more heavily the longer is the relaxation time and the higher is the relaxation temperature. Both processes, the elongation and the relaxation, end with the achievement of the solid state of the hose film at the frost line.
In blown film extrusion, the maximally possible mass output is in most cases limited by the cooling. It can no longer be further increased if in the hose-forming zone, between the exit of the melt from the blow head and the frost line, bubble instabilities arise due to over-high temperatures of the film hose and an accompanying lower melt strength. The air cooling ring is then no longer capable of cooling the then hotter and less stable film bubble in the turbulent air current in such a way that a stable process is established. A further limitation of the maximally possible output, which is attributable to an inadequate cooling, is the jamming of the film in the draw-off mechanism. In this case, the inner side of the film bubble is so warm that, following flattening of the film hose when the film bubble is squeezed through the draw-off rolls, the film ends up sticking. This occurs, in particular, with thicker films.
The object of the present invention is therefore to improve the cooling of the film bubble in blown film extrusion in such a way that the mass output of the blown film plant can be raised.